Denshi Jisho — Online Japanese dictionary

堅 けん 1: (Obscure term) strength; solidity; firmness;
2: (Obscure term) armour; armor
Noun 
米利堅 メリケン 1: (Usually written using kana alone) America; American;
2: fist
Ateji (phonetic) reading, Noun, Noun or verb acting prenominally 
中堅 ちゅうけん 1: main body (of troops);
2: nucleus; backbone; mainstay; key figure;
3: center field; centre field; center fielder; centre fielder;
4: (Noun) athlete competing in the middle-number match in a team competition, i.e. second in 3-on-3, third in 5-on-5 (kendo, judo, etc.)
Common word, Noun, No-adjective 

You can also

Look up kanji details for *堅.

Try a Yahoo Jisho search for *堅 .
Try a Goo Jisho search for *堅 .
Try a Google search for *堅 .